Setting up Local Multiplayer Players

Hi, i'm making my first local multiplayer game and iv come into an issue that i cant seem to find the answer for. Its quite hard to explain... So in the game i have 4 characters and they can all move but with just my keyboard, i have set up random keys for that (going to change it to xbox controller later for each player) anyways i dont know how to sign in or activate the different players in the game, like when you are playing COD for example on split screen and the 2nd player presses 'x' to join that type of stuff, im really not sure how to do that? Also for the character selection im finding it hard to separate it for different players but i was just going to use a bool for that? I'm trying to set my local multiplayer join system like the game 'Crawl' on Steam. Thanks for any help!